The inhibitory effect of antiXAB antisera on the process of TCR indicates that this protein plays a role in the same pathway as the CS proteins.As previously shown, injection of nonimmune sera as well as antibodies against other factors only involved in NER failed to exert inhibition of transcription, in contrast to antisera against various proteins implicated in both NER and transcription initiation. In conclusion, the results of the antiserum microinjection experiments suggest that XAB functions both in TCR and in normal transcription but has no role in GGR.Together these results indicate that XAB is involved in TCR but not in GGR.Since transcription is essential for TCR, it is likely that the observed inhibition of TCR is a consequence of the inhibitory effect of antiXABFL on transcription.However, the antiXABC inhibited the recovery of RNA synthesis after UV irradiation without apparent inhibitory effects on transcription.The molecular mechanism for the coupling of transcription and NER in eukaryotes is unknown.Presumably, a lesion on the transcribed strand is first encountered and marked by an RNA polymerase II elongation complex. CSB was found in vitro and in vivo to reside in an RNA polymerase II complex, probably in an elongation mode. In the present study, we found a dual interaction of part of XAB with a fraction of both CSA and CSB as well as the interaction with XPA.The notion that these interactions are transient may explain our observation that only a small proportion of XAB is bound to CSA and RNA polymerase II. This is consistent with the fact that CSA and CSB appear to reside in different protein complexes. The homology of XAB to SYF may provide a clue for understanding the role of XAB.AntiXABFL antiserum was injected into the cytoplasm of binuclear cells obtained after fusion of normal human fibroblasts.AntiXABFL and antiXABC injections into XPC cells were performed twice and once, respectively.Thus, it is possible that XAB is also involved in the processes associated with cell cycle control and premRNA splicing in mammalian cells.The requirement of XAB in transcription may account for the essential role of SYF and CWF for viability in yeast.The above findings in yeast fit nicely with our observation that a significant proportion of XAB is in a complex with the fraction of RNA polymerase II that is associated with CSB and is thought to be in an elongation mode. Since a tight coupling of transcription elongation and premRNA splicing has been observed, a potential involvement of XAB in premRNA splicing may explain the inhibition of RNA synthesis observed after microinjection of antiXABFL as a consequence of impaired splicing giving rise to arrested transcription.However, it has been reported that transcription may still occur at normal rates in the absence of efficient splicing of nascent premRNA during transcription elongation in human cells. Thus, it is likely that the inhibition of RNA synthesis by antiXABFL resulted from impaired transcription rather than disturbed premRNA splicing.Based on our experimental data and the homology with CWF and SYF, it is likely that XAB is a multifunctional protein involved in cellular processes such as cell cycle control and premRNA splicing as well as TCR and transcription in mammalian cells.